Analog multipliers are known to provide multiplication of an input AC voltage or current. Multipliers generally include a pair of differential amplifier stages which receive their respective DC bias currents from a DC power supply via matched current mirrors and are controlled by independent DC current sources located between the common node of each respective differential amplifier stage and a return of the DC power supply. An operational amplifier is typically employed to provide simultaneous control of the pair of differential amplifier stages which subsequently results in multiplication of the input current with the DC currents provided by the current sources. To produce voltage multiplication, an input voltage is provided to the input of a current multiplier via a resistor that converts the input voltage to an input current. The input current is multiplied by the current multiplier and delivered to the output of the multiplier. The output current is subsequently re-converted back to a voltage via an output operational amplifier to effectively produce voltage multiplication of the input voltage.
Multipliers are commonly used in transmitter compressors and receiver expandors to enable companding of a communication signal transmitted over a radio frequency (RF) channel with limited dynamic range. The companding operation allows wide dynamic range communication signals to be transmitted over limited dynamic range RF channels without significant reduction in the communication signals' signal-to-noise ratios. Compressors reduce, or compress, the dynamic range of the communication signal by a fractional amount dependent upon the dynamic range of the RF channel. In a complementary manner, expandors expand the dynamic range of the received communication signal such that the dynamic range of the original, pre-compressed signal is restored. For example, suppose a communication signal with 100 dB of dynamic range is to be transmitted over an RF channel with a dynamic range of 30 dB. The communication signal would require a 4:1 compression to reduce its dynamic range to 25 dB prior to transmission over the 30 dB dynamic range channel. Upon reception, a 1:4 expansion of the received communication signal would be necessary to restore the original 100 dB of dynamic range.
In most modern medium and high tier wireless and wireline communication systems, companding is a standard system function; however, in low tier, lower cost systems, companding may be an optional system addition. To reduce equipment and service costs, companding may be excluded by system proprietors that are constructing a low tier system. Accordingly, in radios or radiotelephones to be utilized in these low tier systems, compressors and expandors must be removed or bypassed. Typically, to maintain synergy and commonality with the medium and high tier radios and radiotelephones, compressors and expandors are disabled and bypassed in their respective transmitters and receivers by removing the DC power from the multiplier and using analog switching circuitry to reroute the communication signal around the compressor or expandor. However, this switching circuitry requires a significant number of additional circuit components when implemented to bypass a discrete multiplier or a significant amount of additional silicon area when implemented to bypass a multiplier within an integrated circuit. The additional circuit components and silicon area often reduce reliability and limit functionality of the voltage multiplier with which they are associated.
Therefore, a need exists for a method and apparatus that provides a disabled voltage multiplier's input signal a path to the voltage multiplier's output without impacting functionality of the operational voltage multiplier and without introducing a significant number of circuit components or requiring a significant amount of silicon area in an integrated circuit.